The oceans are a vast reservoir of CO2, taking up about two billion tons a year. The rate of this uptake is slowing down, although the yearly amount is increasing sim­ply because there is more and more CO2 in the atmosphere. From 1750 to 1994, 42% of CO2 released went into the sea; but from 1980 to 2005 this figure decreased to 37% as a result of the extra CO2 that we are producing. Carbon dioxide is the gas that bubbles out of soda pop and forms a weak acid when dissolved. The oceans can absorb much more CO2 than by simply dissolving it, however. That gas reacts with H2O to form positive hydrogen ions (H+) and negative carbonate (CO32-) and, mainly, bicarbonate (HCO3-) ions in a “buffering” process. This increases the uptake of CO2 by almost an order of magnitude. The possible increase is quantified by the so-called Revelle buffer factor, which depends on the partial pressure of CO2 at the ocean surface. That is, the more CO2 that is pushing back into the atmosphere, the less CO2 the ocean can absorb out of the air. It takes about a year for these pres­sures to equalize, and it takes thousands of years for carbon in different forms to circulate in the ocean. The CO32 — ion can also combine with calcium to form cal­cium carbonate (CaCO3), the material of coral and some shells. These solids sink into deeper water and stay there for millions of years. If we were to stop producing CO2, it would take 4-10 thousand years for the ocean’s partial pressure of CO2 to get back to normal.

The buffering effect injects much more H+ ions into the ocean than would be created by dissolving CO2 into carbonic acid, and this makes the ocean much more acidic. The ocean is naturally mildly alkaline, with a pH value of 7.9-8.3, and anthropogenic CO2 has decreased it by 0.1 since 1750. This does not sound like a lot, but the number of H+ ions has increased by 30%. Furthermore, computer models predict a decrease between 0.14 and 0.35 in the 21st century. Acid dissolves car­bonate matter such as coral and shells of sea animals, and it can slow or prevent their creation. We have all read about dead or dying coral reefs, though the relation to global warming is conjectural. Phytoplankton, at the bottom of the food chain, absorb almost as much CO2 as plants on land,11 and they are consumed by larger organisms which are the food source of all fishes and whales. Most crustaceans such as krill have chitin rather than carbonate shells, but those that are carbonate-based would suffer from increased acidity. The entire food chain can be upset by acidifi­cation of the oceans. However, there is so far no scientific evidence that this is happening. The 2007 IPCC report states that the effect of increased acidity on marine organisms is poorly known.